The British-born terrorist alleged to have murdered the US journalist James Foley is "close" to being identified by security services, the British ambassador to the United States confirmed.

Sir Peter Westmacott said voice recognition technology had been instrumental in tracing the identity of the man, believed to be a British-born militant from London. Speaking on US television, Mr Westmacott told NBC's Meet the Press: "I cannot say more than this but I know we are close."

It is the first confirmation that the net is closing on Mr Foley's killer, dubbed 'Jihadi John', amid reports the Isis fighter is known to security services.

His comments came as British Foreign Secretary Philip Hammond warned that Isis was almost certain to launch attacks on the UK unless it was stopped. "Isis is turning a swathe of Iraq and Syria into a terrorist state as a base for launching attacks on the West," he said. "Unless it is stopped, sooner or later it will seek to strike us on British soil. We have a vital national interest in tackling the threat."

One of the key suspects reportedly under investigation is Abdel-Majed Abdel Bary (23) a former rapper from west London, who went to Syria last year and later tweeted a picture of himself holding a severed head.

The British Home Secretary Theresa May is expected to propose new terrorism laws this week that will include powers to target radical preachers operating in Britain whose outbursts currently fall short of breaking the law. Other measures include "banning orders" for groups communicating extreme views.

The former shadow Home Secretary David Davis said: "Since these young men are in effect swearing allegiance to a hostile state, they should all forfeit their British citizenship - not just those who are dual nationals."

Up to 500 British citizens are estimated to be fighting in Iraq and Syria and there is growing concern that their return to Britain will pose a threat to national security. The shadow Home Secretary Yvette Cooper said: "More should be done to disrupt the travel plans of those planning to go out to fight through better monitoring of the borders watch list, as well as access to passports. Those returning should face criminal investigations and prosecutions, but they should also be required to engage immediately with the Channel programme, which works to deradicalise people."

US officers from the Federal Bureau of Investigation have arrived in the Midlands to assist British police investigating reports that a Birmingham man may be one of the jihadists holding hostages in Syria.